System on Chip (SoC) Circuits often have embedded clock sources, frequently a phases locked loop (PLL) that generates the clock signal. Although these PLL circuits are the standard way of providing an accurate clock signal, they have some drawbacks, particularly for applications, such as small form factor memory cards like the MicroSD cards, for example. One of these drawbacks is that such devices have very small size boards, so that it becomes difficult to find space for an external crystal oscillator, which is needed as reference clock for PLLs. Another is that if the use of a crystal could be eliminated, the cost saving can be significant for a high volume product. The relaxation oscillator is widely used clock source as an alternative to PLLs. The relaxation oscillator does not need external reference clock, but suffers from large output frequency variation mainly due to process dependence and so requires calibration. This calibration time could be long due to manual intervention and needs costly equipment like high bandwidth oscilloscopes.